SO-CALLED 'BIOLOGICAL PROPERTIES' OF MILK 109 



vestigations have provided us with a considerable amount of 

 information of great interest, and have enlarged our knowledge 

 sufficiently to enable great advances to be made in the prevention 

 of disease, there are still many points which await elucidation. 

 Even where sufficient knowledge has been acquired to be of immense 

 benefit to the community, the precise reactions which are necessary 

 in the body before an individual can be put in a position to resist 

 disease, are to a large extent obscure. 



In order that immunity may be conferred by the mother 

 upon her offspring, she must herself possess immunity to the 

 particular disease or organism which may be under considera- 

 tion. It has been found more convenient by the majority of 

 investigators who were engaged upon the study of inheritance of 

 immunity, to render the mother immune to certain well-known 

 diseases and to study subsequently in the offspring any transference 

 of immunity to this disease. Immunity to a disease thus arti- 

 ficially imposed upon the mother is known as acquired immunity. 

 E.g. vaccination imparts an acquired immunity to smallpox. A 

 person who has once suffered from scarlet fever and other diseases 

 does not usually contract the disease a second time. 



Where an individual or a species possesses immunity to any 

 particular disease without any manipulation being required, such 

 immunity is spoken of as natural immunity. 1 



In addition to these two types of immunity there is yet a third 

 type, known as passive immunity. This form is transitory, and 

 the mechanism employed for its production is different from that 

 of either acquired or natural immunity. 2 It will be shown later 

 that the form of immunity which may be acquired by suckling when 

 the mother herself possesses either natural or acquired immunity, 

 is passive immunity. 



One of the methods employed by the animal organism for 

 protecting itself against the invasion of foreign substances has 

 already been referred to in Chap. Ill, where it was shown that the 

 injection of a foreign protein produces a reaction of the body tend- 

 ing to throw out of solution the invading substance. Analogous 

 methods are employed by the body to deal with invading micro- 

 organisms. Substances are produced which are capable of disinte- 

 grating the bacteria, or which may act by first of all throwing the 

 bacteria into clumps, that is by agglutinating them, and subse- 



1 For example, pigeons enjoy a practical immunity to the bacillus ol 

 human tuberculosis, and rats show a high resistance to anthrax. 



2 Passive immunity is conveyed when an antitoxin found in the body 

 of one animal in response to a stimulus (acquired immunity) is injected into 

 the body of another, for the purpose of neutralising a toxin already present, 

 or to act as a temporary defence against a possible invasion of micro-organisms 

 producing the toxin. Thus where a wound has become infected with material 

 containing the organism of tetanus, an injection of tetanus antitoxin 

 neutralises the toxins formed and gives the body time to develop its own 

 protective mechanism. 



